The present embodiments relate to recording a B0 map of a main magnetic field of a magnetic resonance device in the imaging volume of which an object to be recorded is arranged.
Magnetic resonance imaging (MRI) is based on spins of atomic nuclei aligned in a main magnetic field (B0 field). For many applications, the homogeneity of the main magnetic field (e.g., a field strength that is as constant as possible in the large possible three-dimensional volume) is of importance for the image quality and also for the spatial registration of magnetic resonance images since inhomogeneities may result in distortion. Present-day superconducting main field magnets enable main magnetic field homogeneities with deviations of less than 1 ppm across a volume of 30 to 40 cm ball diameter, which may then be typically referred to as the homogeneity volume of the magnetic resonance device. Inhomogeneity problems may, for example, occur when outer regions of the anatomy of a patient (e.g., the shoulder) are to be recorded, since, due to the lack of space in the patient receiving region of a magnetic resonance device, these may not be positioned centrally.
The body of a patient introduced into the main magnetic field generates additional inhomogeneities. Human tissue has a relative magnetic permeability that differs from one. For example, discontinuities between air and tissue produce significant distortion of the main magnetic field. The inhomogeneous distribution of water/air/bone/fat in the human body also results in another distortion of the main magnetic field that is different for each patient.
While inhomogeneities of the main magnetic field induced by the magnetic resonance device and the environment thereof may be corrected by permanent measures (e.g., static shim devices), dynamic shim devices are used for main magnetic field inhomogeneities caused by the introduced object in order to restore homogeneity to the greatest extent possible. Thus, the influence of dynamic shim devices on the main magnetic field may be adjusted, where generally shim coils are used as dynamic shim devices. Herein, shim coils may be used as part of the gradient coil or also as local shim coils (e.g., as part of local coil arrangements).
Parameterization of dynamic shim devices for a specific patient (e.g., to determine suitable currents through the shim coils) requires knowledge of the status of the main magnetic field (e.g., the inhomogeneity). It is known to calculate a B0 map (e.g., main magnetic field map) that may describe the B0 distribution directly or indirectly (e.g., by phase differences) by the magnetic resonance device with a patient already positioned therein. With a specific spatial resolution (e.g., in the range of two to ten millimeters), inhomogeneity is determined three-dimensionally in the imaging volume or in the homogeneity volume (e.g., by calculating the local Larmor frequency). The calculation of the B0 map is performed as part of the “adjustment” before the actual imaging scan and is comparatively time-consuming (e.g., ranging from 15 to over 30 seconds). As part of the adjustment, then, for example, shim parameters (e.g., shim currents) are also ascertained and set.
Known procedures calculate the greatest possible volume or the entire homogeneity volume of the magnetic resonance device in order to calculate the B0 map. Herein, the examination object or a proportional occupation of the homogeneity volume may be acquired completely at the specified position of a patient table of the magnetic resonance device so that the B0 map provides sufficient information for each arbitrary adjustment volume (e.g., in which the homogeneity is to be established and which may be requested subsequently from an imaging scan).
For example, with the double-echo steady state (DESS) shim, with which the calculation of the B0 map is based on a DESS magnetic resonance sequence as a map recording sequence (e.g., a FADE sequence (fast double echo)), a volume of approximately 500 mm×450 mm×450 to 500 mm is acquired. This corresponds to the complete homogeneity volume of the magnetic resonance device minus a constraint in the vertical direction due to the patient bench. In the majority of cases, an unnecessarily large volume is acquired, thus resulting in a loss of time.
Special gradient-echo based main field map scans with adjusted volumes and resolutions have also been suggested for special applications, thus achieving a small saving on scan time or enabling the same for resolutions. However, this only achieves an adjustment for special imaging sequences.
It is not possible to achieve a saving on scan time by automatically constraining the recording region for the B0 field map to the recording region of a subsequent imaging sequence, since the coverage of the B0 map is not sufficient for a subsequent imaging scan with a larger/different recording region, and this would require a completely new recording.
The scan time is generally dominated by the repetition time (TR), the number of slices to be recorded, and the number of phase-encoding steps. In the case of map recording sequences for calculating a B0 field map in a recording region, the repetition time is selected as minimal and is oriented with respect to the necessary echo times of the corresponding map recording sequence so that the repetition time may not be adjusted as a free parameter. Hence, in the case of a more or less fixed image element size, the number of image elements in the phase-encoding direction and the number of slices are effectively relevant for the scan time. This applies analogously to three-dimensional magnetic resonance sequences, where instead of slices, there is a second phase-encoding direction.